In recent years, there has been an increased demand for high temperature oxygen sensors, mainly for the monitoring and control of combustion processes, such as the combustion of hydrocarbons in an internal combustion engine. One device of this type widely used for automative engine control is an electrochemical oxygen concentration cell, usually made of zirconia (ZrO.sub.2). In the most common configuration of this device, the ZrO.sub.2 electrolyte is in the form of a thimble with one side exposed to the combustion environment and the other exposed to air as a reference atmosphere. This device provides an EMF output which is proportional to the logarithm of the oxygen partial pressure in the combustion environment.
Despite its low sensitivity, this device is widely used on automobiles to control and maintain the air-to-fuel mixture in the engine cylinder at the stoichiometric value. A stoichiometric mixture contains just enough oxygen to burn the fuel completely to carbon dioxide and water. The satisfactory operation of this device arises from the fact that the oxygen partial pressure in the product of combustion (exhaust gas) changes by many orders of magnitude (for example, twenty) as the air-to-fuel mixture is varied through the stoichiometric value.
On the other hand, for the purpose of reducing fuel consumption, it is generally desirable to operate internal combustion engines with "lean" air-to-fuel mixtures, which contain excess air. For these lean mixtures, the oxygen partial pressure after combustion exhibits only a small and gradual change with change in the air-to-fuel mixture. These small changes cannot be easily measured with the above-mentioned device. One approach for obtaining high sensitivity devices for use in lean air-to-fuel operation is to employ a so-called oxygen-pumping scheme. Such oxygen-pumping is based on the fact that if a current is passed through an oxygen-conducting electrolyte (e.g. zirconia), oxygen is transferred (pumped) from one side of the electrolyte to the other. Several oxygen sensors based on this principle have been described in the prior art. Examples are those described in U.S. Pat. Nos. 3,923,624 to Beckman et al; 3,654,112 to Beckman et al; 3,907,657 to Heijne et al; and 3,698,384 to Jayes.
Recently a series of U.S. patents to Hetrick and Hetrick et al. (U.S. Pat. Nos. 4,272,320; 4,272,330; 4,272,331) describe an oxygen-pumping device that has improved characteristics over previously described devices, e.g., higher speed of response, lower sensitivity to temperature variations and independence from ambient total pressure changes. These features make this device particularly useful for automotive engine use. This device has two pieces of dense zirconia sealed together to form a cavity that communicates with the outside volume through one or more apertures. Electrodes are deposited on the inside and outside walls of each of the two sections of the device, thus forming an oxygen pumping cell and a sensing cell. The fabrication of this device involves several steps, one step being the sealing of the device with glass frit. Having to seal pieces of zirconia in the fabrication of such a prior art device presents problems with respect to durability and cost. It would be desirable to avoid the need for sealing an enclosed volume. It would also be desirable to use fewer electrodes. In addition to simplified fabrication, it would be desirable to increase reproducibility and reliability and have operation occur at a lower temperature. These are some of the problems this invention overcomes.